Surface imaging utilizing exoelectron emission

ABSTRACT

A system for obtaining an image of a surface by utilizing exoelectron emission is described. Surface characteristics such as homogeneity, microscopic flaws and discontinuities in the work function and defect distribution across the surface of the sample can be imaged by detecting the exoelectrons which are emitted from the surface under controlled conditions. The system includes a channel electron multiplier which is positioned in the proximity of the sample under study. The sample is heated to a temperature which enhances exoelectron emission. Exoelectron emission can also be stimulated by illuminating the substance with light or by stressing, cutting, or impacting the material. The emitted exoelectrons are detected by the multiplier and the multiplier emits secondary emission electrons in response to the exoelectrons. A phosphor screen receives the secondary emission electrons and yields a visual output in response thereto. Because the exoelectrons strike the multiplier at positions corresponding to positions from which they emanated from the sample surface, a visual image of the surface is obtained at the output screen. Exoelectron emission from a surface is dependent upon the characteristics of the surface, and therefore flaws and variations in the homogeneity of the surface are evident in the visual image.

United States Patent [191 Braunlich 51 Feb. 6, 197-3 [54] SURFACE IMAGING UTILIZING EXOELECTRON EMISSION [75] Inventor: Peter F. Braunlich, Bloomfield Hills,

Mich.

[73] Assignee: The Bendix Corporation [22] Filed: May 13, 1971 [21] App]. No.: 142,967

[52 US. Cl ..250/71 R, 250/83 R, 250/833 R, 250/213 VT [51] Int. Cl ..G0lt l/ll, 1101 31/50 [58] Field of Search.250/71 R, 83 R, 83.3 R, 213 VT [56] References Cited UNITED STATES PATENTS 3,030,514 4/1962 Salinger ..250/2l3 VT 3,449,582 6/1969 Sackinger..... 3,484,610 12/1969 Becker 3,612,868 10/1971 Becker ..250/83 R Primary Examiner-Archie'R. Borchelt Att0rneyLester L. Hallacher and Plante, Hartz, Smith & Thompson ABSTRACT A system for obtaining an image of a surface by utilizing exoelectron emission is described. Surface characteristics such as homogeneity, microscopic flaws and discontinuities in the work function and defect distribution across the surface of the sample can be imaged by detecting the exoelectrons which are emitted from the surface under controlled conditions. The system includes a channel electron multiplier which is positioned in the proximity of the sample under study. The sample is heated to a temperature which enhances exoelectron emission. Exoelectron emission can also be stimulated by illuminating the substance with light or by stressing, cutting, or.impacting the material. The emitted exoelectrons are detected by the multiplier and the multiplier emits secondary emission electrons in response to the exoelectrons. A phosphor screen receivesthe secondary emission electrons and yields a visual output in response thereto. Because the exoelectrons strike the a multiplier at positions corresponding to positions from which they emanated from the sample surface, a visual image of the surface is obtained at the output screen. Exoelectron emission from a surface is dependent upon the characteristics of the surface, and therefore flaws and variations in the homogeneity of the surface are evident in the visual image.

17 Claims, 2 Drawing Figures 'ferent SURFACE IMAGING UTILIZING EXOELECTRON EMISSION BACKGROUND OF THE INVENTION ous forms of radiation. Exoelectron emission also occurs when a substance is distorted by the application of a force, such as bending, twisting, or impacting. The emission occurs while the distortion is taking place and continues thereafter, although it decays withtime.

. Cutting, milling, grinding, and all other processes which remove material also enhance exoelectron emission.

The phenomenon of thermally stimulated exoelectron emission is different from thermionic emission because it occurs at a much lower temperature; and the phenomenon of optically stimulated emission is diffrom photoemission, inasmuch as the wavelengths of the stimulating light are longer than the threshold wavelengths for photoemission. It is believed that exoelectron emission occurs because the exposing of the substance to radiation such as ultraviolet, X-ray, alpha or beta particles, or other similar types of radiation causes electrons within the material to be raised above the Fermi escape level and be trapped along a thin surface layer of the substance. Upon heating to temperatures which are in excess of ambient, whichare less than the higher temperatures required for thermionic emission, the trapped electrons escape across the potential barrier at the surface of the material into the gas or vacuum atmosphere adjacent the substance surface. A detailed description of the emission of exoelectrons is presented in an article by K. Becker entitled Principles of Thermally Stimulated Exoelectron Emission (TSEE) Dosimetry.

Studies of exoelectron emission of various substances indicates that peak emission occurs at specific temperatures. For example, maximum exoelectron emission for lithium fluoride occurs at a temperature of approximately 150 centigrade. Accordingly, by elevating a sample of lithium fluoride to 150 centigrade the emission of exoelectrons can be maximized.

SUMMARY OF THE INVENTION Because exoelectron emission is dependent upon the surface of the dielectric sample under study it would intuitively appear that the emission of exoelectrons along the surface would vary in accordance with surface variations. These variations could include surface faults such as cracks or pits of microscopic dimensions and which accordingly are invisible to the naked eye or to an optical microscope. The surface variations could also include small pockets of material impurities which constitute variations of the otherwise homogeneous character of the sample. Density variations also cause a oel'ectron emission occasioned by variations across the surface of the substance.

The inventive system is directed to a technique for utilizing the variations in exoelectron emission to detect microscopic or macroscopic flaws as well as variations in the homogeneity of the surface by yielding information which is indicative of the exoelectron emissionoccurring at all points along the surface. This is ac- 1 complished by placing a channel electron multiplier in the vicinity of the surface. As a consequence, the emitted exoelectrons impact the channel multiplier and are increased in number by an order of several magnitudes. Because of the operation of the channel multiplier, the electron increase occurs on a point-to-point basis, and as a consequence the number of electrons emanating from a particular point of the channel multiplier is indicative of the number of exoelectrons which impinge upon a corresponding point on the channelmultiplier. The number of electrons emanating from a particular point of the channel multiplier is therefore indicative of the number of exoelectrons emanating from a corresponding point on the surface of the sample under study. The secondary emission electrons emanating from the output surface of the channel multiplier are directed to a phosphor screen which yields a visible image in response to the impact of the electrons. As a consequence, the variations in the surface characteristics of the sample under study are represented by a visual output on the phosphor screen. Thisresults in a'visible indication of the surface variations of the sample so that ordinarily invisible structural incongruities and nonhomogeneous features are visually indicated in the image.

If desired, a particular area of the visual image can be magnified by the use of electrostatic or magnetic lenses so that a detailed study of the surface of the sample can be made.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a preferred embodiment of the inventive system.

FIG. 2 is the exoelectron emission curve of lithium fluoride.

DETAILED DESCRIPTION The preferred embodiment shown in FIG. 1 is used to image the surface characteristics of a Sample Sub stance 10 by detecting the emission of exoelectrons from the substance and forming an image in .accordance with the emission pattern of these electrons. In the preferred embodiment described, thermally stimulated exoelectrons are used; any type of stimulation canbe used within the scope of the invention, which is directed to imaging the emitted exoelectrons and not merely counting them. Sample 10 can bemade from any type of material such as, for example, metal, glass, sapphire, ruby, alkali halides, and various nonlinear optical crystals. Before being placed on Heater 11, Sample 10 is exposed to radiation such as X- ray, gamma rays, alpha or beta particles, or other type of radiation which causes electrons within the substance torise above the Fermi level and be trapped along the surface layer of the substance.

After being exposed to the appropriate type of radiation, Sample 10 is placed in physical and heat conducmicrochannel plate. As is known, a microchannel plate is a thin wafer of material containing a large number of channels passing completely through the wafer. The channels act as electron multipliers so that an electron impinging upon the Input Surface 13 of the Microchan- 'nel Plate causes the emission of secondary electrons within one of the channels. The number of secondary emission electrons is greatly in excess to the single electron which causes the initial emanation of secondary electrons. As a consequence, at the Output Surface 14 of each channel a large number of electrons are emitted for each electron impacting Input Surface 13. However, because the emission of secondary electrons occurs within the individual channels, any image-information present' in the radiation impacting on Surface 13 will be retained by the electrons emanating from output Surface 14. The electronsemanating from output Surface 14 impact a Screen 15 which is made of a material which fluoresces when impacted with electrons. As an example, this screen can be made from a phosphor. Because of the luminescence occasioned by the impact of electrons, an image which is indicative of the image information contained within the exoelectrons emanating from Sample is reproduced on Screen as a visual image. Because the-emission of exoelectrons is dependent upon the surface characteristics of Sample 10, the visual image present on Screen 15 will likewise be dependent upon the surface characteristics of Sample 10. As a consequence, inhomogeneous characteristics within the Sample 10 can be detected because of the variation in exoelectron emissioncaused by these variations. Also, surface flaws such as microscopic or macroscopic cracks or pits will also be present in the visual image present on Screen For best results it is preferable to operate the system in an evacuated atmosphere. Accordingly, an Evacuable Casing 16 is provided. Sample 10, Microchannel Plate 12, and the input'surface of phosphor Screen 15 are positioned in Casing 16 which is then evacuated so that no appreciable atmosphere exists around these elements. As a consequence, electrons flowing from Sample 10 to Microchannel Plate 12 and electrons flowing from Microchannel Plate 12 to Output Screen 15 do not. impact with molecules of gas, and a large percentage of the electrons emanating from the various elements are detected by the succeeding element.

The operation of the system also is dependent upon the attraction of the electrons from one surface to another. For this reason a series of Biasing Terminals 17 through are provided. A voltage V is applied across Terminals l7 and 18 so that Input Surface13 of Microchannel Plate 12 is more positive than Sample 10. Exoelectrons emanating from Sample 10 are therefore attracted to the Input Surface 13 of MCP 12. In order to enhance the propagation of electrons through the channels of MCP l2 and to cause the emission of secondary electrons, a biasing voltage V, is applied to Terminals l8 and 19 which are respectively connected to the Input Surface 13 and Output Surface 14 of MCP 12. Electrons are therefore attracted through the MCP l2 and secondary emission electrons emanate from Output Surface 14. These electrons are attracted to Output Screen 15 by the use of a third biasing voltage V applied so that Screen 15 is more positive than Output Surface 14 of MCP 12. This is accomplished by applying the biasing voltage V across biasing Terminals l9 and 20. Although the biasing voltage V V and V will be dependent upon the characteristics of the elements used, particularly the MCP 12 and Output Screen 15, exemplary voltages are 300 volts for V 1,000 volts for V and 4,000 volts for V It will be appreciated that the relative sizes and dimensions of the system shown in FIG. 1 are not proportional to those of an actual system. In an actual system, the thickness of MCP12 will be very small and probably will be less than 0.01 inch. Also the distance between Sample 10 and MCP 12 and the distance between MCP 12 and Output Screen 15 will be much less than those shown in the drawings and will be on the order of 0.1 inch.

It is now evident that the use of a microchannel plate to receive and multiply the exoelectrons emanating from Sample 12 is highly advantageous over the use of standard electron multipliers as used in the prior art systems. This is so because image information present in the exoelectron radiation pattern is preserved and reproduced on the output screen, thereby making it possible to visually reproduce the radiation pattern of the exoelectrons. As a consequence, surface variations evidenced by the variation of emission of exoelectrons can be visually observed. If desired, a very small area of the visual Image 15 can be magnified by the use of electrostatic and magnetic lenses so that surface characteristics which are too small for the, naked eye to view can be studied. 1

The ability to visually observe the surface characteristics of a sample without destroying or otherwise harming the surface is highly advantageous in many areas. For example, laser systems frequently suffer optical damage of the various optical components when high-powered laser beams are used. Heretofore it has been extremely difficult, if not impossible, to determine the presence of laser damage which may be detrimental to the intended operation of the laser system but which is not otherwise easily detected. The invention makes it possible to detect and locate such flaws without damaging the optical component.

Inhomogeneous variations of the structural characteristics of a material can be detected by the inventive system because these variations cause changes in the exoelectron emission. As a consequence it is possible to nondestructively test lenses and other elements to determine their quality and thus determine whether or not they are of the quality required for the intended usage. Also, having detected various surface flaws by the inventive system it is possible to determine whether or not the tested elements can be repaired or polished so that the flaws are no longer detrimental to the system in which they are intended to be used.

The embodiment described with respect to FIG. 1 includes a single microchannel plate in which the channels are perpendicular to the Input and Output Surfaces 13 and 14, respectively. It should be noted that the single Microchannel Plate 12 can be replaced by a compound type of Microchannel Plate. In the compound Microchannel Plate two individual plates are fabricated such that the channels passing through the plates are angularly disposed with respect to the input and output surfaces of the plates. The two plates are then placed together so that the output surface of one plate is against the input surface of the other. The plates are positioned so that the channels of the two plates slope in opposite directions. This type of com pound microchannel plate has a gain which is approximately times larger than the gain of a single MCP. .As a consequence when large electron gains are required the compound type of microchannel plate may be preferable.

FIG. 2 shows the known exoelectron emission curve for a sample of lithium fluoride and is useful inunderstanding the concept of exoelectron-emission. In FIG. 2 the maximum exoelectron emission of approximately 2,750 counted electrons at a temperature of about 150 centigrade is indicated by Peak 22. The exoelectron emission drops rapidly above 150 until a second Peak 23 occurs at approximately 205 centigrade. It will be noted that the second Peak 23 is substantially lower thanPeak 22 which occurs at 150. The

. ration shown in FIG. 1, Heater llwill be energized so that the Sample 10 is raised to a temperature of 150 centigrade. At this point the system is actuated by the application of the voltages so that a visual image of the surface .of Sample 10 isproduced on Output Screen 15.

Surface variations in Sample 10 will then be indicated by the variation of emission of exoelectrons occasioned by these variations. As a consequence, the point-topoint exoelectron output will vary substantially from the peak output shown inFIG. 2 at the 150 tempera ture, and the areas on the surface of Sample 10 which contain flaws and structural incongruities will be revealed in the image present on OutputScreen 15.

What is claimed is:

l. A system for imaging surface characteristics of a substance by detecting exoelectron emission from said substance comprising:

means for causing said substance to emit exoelectrons, said substance emitting different quantities of exoelectrons at different surface areas in accordance with variations in said surface areas; channel electron multiplier means in the proximity of said substance, said multiplier means receiving exoelectrons at points corresponding to points of emanation from. the surface of said substance so that said different areas cause different exoelectron inputs to said multiplier means and surface image information represented by said exoelectrons is present in the output of said multiplier means, the output of said multiplier means being a plurality of output electrons which greatly exceeds the total number of exoelectrons impacting said multiplier means;

and output means in the proximity of said multiplier for detecting said output electrons, said output means receiving said output electrons at points corresponding to said points on said substance surface so that image information present in said exoelectrons is preserved at said output means, said output means being composed of a material which yields a visible indication when impacted with electrons so that a visual image of said substance surface is produced at said output means.

2. The system of claim 1 wherein said multiplier means is a microchannel plate.

3. The system of claim 2 wherein said multiplier means is a compound microchannel plate formed from at least two single microchannel plates each having channels angularly disposed with respect to the input and output surfaces, said two microchannel plates being joined along a common plane and the slopes of said channels with respect to said common plane being opposite.

4.-The system of claim 1 wherein said substance, said multiplier means and said output means are positioned in an evacuated casing;

and further including first means for placing said substance at a voltage V,, second means for placing a voltage V, across said multiplier means, and third means for placing said output means at a voltage V said voltages V V and V being different so that v v, V,.

5. The system of claim 2 wherein said output means is a fluorescent screen and further including means for placing said substance, said multiplier means and screen at voltages of V V and V respectively, said voltages being positive and having potentials so that V V V,.

6. The system of claim 1 wherein said substance has an exoelectron emission characteristic having at least one peak of exoelectron emission at a particular temperature below the thermionic temperature of said substance;

and wherein said means for causing includes temperature control means for bringing said substance to said particular temperature.

7. The system of claim 1 wherein said means for causing includes a light source for illuminating said substance to thereby stimulate said exoelectron emissron.

8. The system of claim 1 wherein said means for causing includes means for applying at least one force to said substances to thereby stimulate said exoelectron emission.

9. A method of imaging surface characteristics of a substance including the steps of:

subjecting said substance to radiation to thereby raise electrons above the Fermi level of said substance and trap said electrons as exoelectrons along surface levels of said substance;

placing said substance in the proximity of a microchannel plate means;

controlling the temperature of said substance so that a particular temperature at which maximum exoelectron emission occurs is maintained, the exoelectron emission of said substance varying from point to point along the surface of said substance in accordance with variations in said surface;

attracting said exoelectrons to said microchannel plate to produce output electrons,

said output electrons emanating from said multiplier at points corresponding to the points of emission of said exoelectrons from said surface so that there is a point-to-point correspondence between said exoelectrons and said output electrons and surface image information represented by the distribution of said exoelectrons is present in the distribution of said output electrons;

detecting said output electrons with detection means which fluoresce in response to impingement of electrons, said detection occurring with a point-topoint correspondence with said exoelectrons so that a visual image of said substance surface is produced by said detection means. i

10. The method of claim 9 wherein said substance has an eXoelectron emission characteristic which has a maximum exoelectron emission peak at a temperature below the thermionic emission temperature of said substance and said temperature controlling step includes raising the temperature of said substance to said particular temperature.

11. The method of claim 10 further including the 7 step of placing said substance, said microchannel plate angularly disposed with respect to the plane of the 13. A method of imaging surface characteristics of a substance including the steps of:

increasing the emission of exoelectrons from said substance;

placing said substance in the proximity of channel electron multiplier means;

attracting exoelectrons emitted from said substance to said multiplier means on a point-to-point basis so that electrons emanating from a particular point on said substance impact said multiplier means at a corresponding point, and said multiplier emits a plurality of multiplied electrons for each impacting electron at points corresponding to said substance points so that image information present in the exoelectron distribution is present in said multiplied electrons;

detecting said multiplied electrons on a point-topoint basis with detection means capable of yielding visual responses to electrons and thereby producing a visual image of said image informa tion. 14. The method of claim 13 wherein the step of increasing emission of exoelectrons includes the steps of: sub ecting said substance to radiation to raise electrons above the Fermi level of said substance and trap said electrons as exoelectrons; and maintaining the temperature of said substance at a particular temperature at which maximum exoelectron emission occurs. 15. The method of claim 13 wherein the step of increasing exoelectron emission includes the step of:

illuminating said substance with electromagnetic radiation and thereby enhancing exoelectronemission. 16.The method of claim 13 wherein the step of increasing exoelectron emission includes the step of:

subjecting said substance to a force and thereby enhancing exoelectron emission. 17. The method of claim 13 wherein the step of increasing exoelectron emission includes the step of:

. removing a portion of said substance and thereby increasing the exoelectron emission. 

1. A system for imaging surface characteristics of a substance by detecting exoelectron emission from said substance comprising: means for causing said substance to emit exoelectrons, said substance emitting different quantities of exoelectrons at different surface areas in accordance with variations in said surface areas; channel electron multiplier means in the proximity of said substance, said multiplier means receiving exoelectrons at points corresponding to points of emanation from the surface of said substance so that said different areas cause different exoelectron inputs to said multiplier means and surface image information represented by said exoelectrons is present in the output of said multiplier means, the output of said multiplier means being a plurality of output electrons which greatly exceeds the total number of exoelectrons impacting said multiplier means; and output means in the proximity of said multiplier for detecting said output electrons, said output means receiving said output electrons at points corresponding to said points on said substance surface so that image information present in said exoelectrons is preserved at said output means, said output means being composed of a material which yields a visible indication when impacted with electrons so that a visual image of said substance surface is produced at said output means.
 1. A system for imaging surface characteristics of a substance by detecting exoelectron emission from said substance comprising: means for causing said substance to emit exoelectrons, said substance emitting different quantities of exoelectrons at different surface areas in accordance with variations in said surface areas; channel electron multiplier means in the proximity of said substance, said multiplier means receiving exoelectrons at points corresponding to points of emanation from the surface of said substance so that said different areas cause different exoelectron inputs to said multiplier means and surface image information represented by said exoelectrons is present in the output of said multiplier means, the output of said multiplier means being a plurality of output electrons which greatly exceeds the total number of exoelectrons impacting said multiplier means; and output means in the proximity of said multiplier for detecting said output electrons, said output means receiving said output electrons at points corresponding to said points on said substance surface so that image information present in said exoelectrons is preserved at said output means, said output means being composed of a material which yields a visible indication when impacted with electrons so that a visual image of said substance surface is produced at said output means.
 2. The system of claim 1 wherein said multiplier means is a microchannel plate.
 3. The system of claim 2 wherein said multiplier means is a compound microchannel plate formed from at least two single microchannel plates each having channels angularly disposed with respect to the input and output surfaces, said two microchannel plates being joined along a common plane and the slopes of said channels with respect to said common plane being opposite.
 4. The system oF claim 1 wherein said substance, said multiplier means and said output means are positioned in an evacuated casing; and further including first means for placing said substance at a voltage V1, second means for placing a voltage V2 across said multiplier means, and third means for placing said output means at a voltage V3, said voltages V1, V2, and V3 being different so that V1 < V2 < V3.
 5. The system of claim 2 wherein said output means is a fluorescent screen and further including means for placing said substance, said multiplier means and screen at voltages of V1, V2, and V3, respectively, said voltages being positive and having potentials so that V1 < V2 < V3.
 6. The system of claim 1 wherein said substance has an exoelectron emission characteristic having at least one peak of exoelectron emission at a particular temperature below the thermionic temperature of said substance; and wherein said means for causing includes temperature control means for bringing said substance to said particular temperature.
 7. The system of claim 1 wherein said means for causing includes a light source for illuminating said substance to thereby stimulate said exoelectron emission.
 8. The system of claim 1 wherein said means for causing includes means for applying at least one force to said substances to thereby stimulate said exoelectron emission.
 9. A method of imaging surface characteristics of a substance including the steps of: subjecting said substance to radiation to thereby raise electrons above the Fermi level of said substance and trap said electrons as exoelectrons along surface levels of said substance; placing said substance in the proximity of a microchannel plate means; controlling the temperature of said substance so that a particular temperature at which maximum exoelectron emission occurs is maintained, the exoelectron emission of said substance varying from point to point along the surface of said substance in accordance with variations in said surface; attracting said exoelectrons to said microchannel plate to produce output electrons, said output electrons emanating from said multiplier at points corresponding to the points of emission of said exoelectrons from said surface so that there is a point-to-point correspondence between said exoelectrons and said output electrons and surface image information represented by the distribution of said exoelectrons is present in the distribution of said output electrons; detecting said output electrons with detection means which fluoresce in response to impingement of electrons, said detection occurring with a point-to-point correspondence with said exoelectrons so that a visual image of said substance surface is produced by said detection means.
 10. The method of claim 9 wherein said substance has an exoelectron emission characteristic which has a maximum exoelectron emission peak at a temperature below the thermionic emission temperature of said substance and said temperature controlling step includes raising the temperature of said substance to said particular temperature.
 11. The method of claim 10 further including the step of placing said substance, said microchannel plate and said detection means in an evacuated atmosphere.
 12. The method of claim 10 further including the step of utilizing a compound microchannel plate in place of said microchannel plate means, said compound microchannel plate being formed of at least two microchannel plates each having a plurality of channels angularly disposed with respect to the plane of the plates, said two plates being adjacently positioned so that the slopes of said channels are oppositely disposed with respect to the plane of said compound microchannel plate.
 13. A method of imaging surface characteristics of a substance including the steps of: increasing the emission of exOelectrons from said substance; placing said substance in the proximity of channel electron multiplier means; attracting exoelectrons emitted from said substance to said multiplier means on a point-to-point basis so that electrons emanating from a particular point on said substance impact said multiplier means at a corresponding point, and said multiplier emits a plurality of multiplied electrons for each impacting electron at points corresponding to said substance points so that image information present in the exoelectron distribution is present in said multiplied electrons; detecting said multiplied electrons on a point-to-point basis with detection means capable of yielding visual responses to electrons and thereby producing a visual image of said image information.
 14. The method of claim 13 wherein the step of increasing emission of exoelectrons includes the steps of: subjecting said substance to radiation to raise electrons above the Fermi level of said substance and trap said electrons as exoelectrons; and maintaining the temperature of said substance at a particular temperature at which maximum exoelectron emission occurs.
 15. The method of claim 13 wherein the step of increasing exoelectron emission includes the step of: illuminating said substance with electromagnetic radiation and thereby enhancing exoelectron emission.
 16. The method of claim 13 wherein the step of increasing exoelectron emission includes the step of: subjecting said substance to a force and thereby enhancing exoelectron emission. 